Endogenous DHP-sensitive Ca(2+) channels in Pleurodeles oocytes.

The double electrode voltage-clamp technique was used to study voltage-dependent Ca(2+) channels in Pleurodeles oocytes. From a holding potential of -80 mV, Ba-current (IBa) (recorded in Cl-free solution, Ba(2+ = 40 mM) activated at -36.7 +/- 4 mV, peaked at -11.6 +/- 4 mV and reversed at 55 +/- 7 mV (n = 24). This current activated slowly (rise time was 0.98 +/- 0.2 s;n = 14 at -10 mV) and was not inactivated. Cadmium (Cd(2+), 500 microM) completely inhibited I(Ba). The effect of Cd(2+) was dose-dependent (EC(50) = 37 +/- 5 microM; n = 5). Moreover, IBa was insensitive to omega-conotoxin (10 microM) but interestingly this I(Ba) displayed dihydropyridine (DHP) sensitivity. Bay K 8644 (5 microM), a DHP activator, increased the peak current amplitude in a dose-dependent manner (EC(50) = 5.9 +/- 0.6 microM; n = 10) and shifted the threshold and the maximum of current/voltage relationship towards negative potentials by -10 mV. Nifedipine (5 microM), a DHP antagonist, decreased I(Ba) by 80% at HP of -80 mV (EC(50) = 1.2 +/- 0.2 microM; n = 6). We concluded that Pleurodeles oocytes possess High-Voltage Activated Ca(2+) channels with properties similar to L-type Ca(2+) channels.